1. Field of the Invention
The present invention relates generally to an antenna structure, and more particularly to a planar log-periodic antenna using a coplanar waveguide (CPW)-fed technique.
2. Description of Related Art
As communication technology progresses and develops, communication products have become the most economical messaging devices that offer the most extensive range. While the communication products are getting increasingly relied on, more attention will be paid to its mobility and convenience. To overcome the drawbacks in wire communication networks such as inconvenient hardware architecture layout and limited application, wireless communication techniques have been accordingly born.
Wireless communication products have been widely used in modern life as public communication devices or private portable devices. Therein, antennas play an important role in transmitting and receiving radio waves. An antenna is a coupling component or a conductive system through which conversion between an electrical signal in circuit and electromagnetic energy in space can be achieved. When transmitting a signal, the antenna converts radio frequency electrical energy into electromagnetic energy that is further radiated to the surrounding environment. After receiving a signal, the antenna converts electromagnetic radiation into radio frequency electrical energy to be processed by a receiver. When the frequency of a radio signal fed into a transmission line varies, the impedance of the antenna will vary accordingly. As a result, all incident energy can be radiated out from the antenna at a resonant frequency if an appropriate signal feed method and impedance matching are considered. Besides, antenna design will be different according to the type of communication specifications and techniques.
A log-periodic antenna is a frequency-independent antenna with stable energy gain, which can efficiently receive and transmit broadband energy but provides less gain compared with narrow band antennas. The log-periodic antenna, suitable for medium and short wave communication, is able to be applied to various frequencies and elevation angles as well as exhibit directivity. Since the log-periodic antenna has stable input impedance and radiation pattern in the operating frequency band, the log-periodic antenna is often used in electromagnetic compatibility testing.
Among various types of antennas, a planar antenna is the most commonly used one. Due to the characteristics of their small volume, light weight, low cost, high reliability, and easiness to fabricate and attach to any object, planar antennas such as microstrip antennas and printed slot antennas have been significantly applied to wireless communication systems.
A planar log-periodic antenna is a new type of the log-periodic antenna manufactured through etching the surface metal of a substrate. The antenna has a planar structure that is directly formed on a printed circuit board and thus offers advantages of the general planar antennas. However, resonant current of such a structure generates energy in two orthogonal directions, i.e., cross-polarized radiation produced on an X-axis and a Y-axis, thereby resulting in energy loss inside the antenna and leading to low radiation gain on a Z-axis.
Therefore, there is a need to provide a planar log-periodic antenna that provides small volume, low cost, easiness to fabricate and ability to suppress undesired cross-polarized radiation during energy transmission.